1. Field of the Invention
The present invention relates to conductive liquid crystalline polymer composites and to a process for the preparation thereof, and also the preparation of conductive liquid crystal alignment layers for liquid crystal displays and to a process for the preparation thereof. More particularly, the present invention relates to the preparation of conductive liquid crystalline polymer composites with excellent processibility and thermal stability and to a process for the preparation of conductive liquid crystal alignment layers that reduce static charges and dust contamination at the time of manufacturing liquid crystal display devices.
2. Description of the Prior Art
Flat-panel displays using liquid crystals have been widely employed by a variety of electronic devices such as calculators, computers, and televisions. These liquid crystal display (LCD) has also received a great deal of investigation and attention due to their potential applications to high technological products requiring man-machine interface. Liquid crystals in the display elements of these device are usually oriented by means of an alignment layer obtained by various techniques. Alignment is generally obtained by means a rubbing, stretching, or shearing technique, or by LB deposition (Chem. Lett. 1991, 1351).
However, those alignment techniques are in some cases not useful for industrial applications. Furthermore, they involve high expenses or do not maintain bistability. In particular, the rubbing technique, in which an insulating polymer film is rubbed with cloth, generate static charges, which can trap dust particles between the cell surfaces, and this has several serious problems related thereto. For example, poor alignment, severe wedging of the cell, and electrical breakdown across the dust particle reduces the life time of the cell and disrupts the optical guided mode structure.
Therefore, it is necessary to develop a conductive alignment layer in order to avoid the production of static charges during the fabrication of liquid crystal displays. For example, one method was introduced in Japanese laid open Patent No. 3-234919 that orients the liquid crystals by rubbing an electrically conducting polyaniline film prepared on an ITO glass via electrochemical polymerization.
According to the patent there are several advantages in the use of the conductive polymer membrane: (a) it removes static charges which has been a problem in the case of insulating polymer alignment layers, and (b) it reduces applying voltage of the display cell due to the conductivity of the polymer.
Despite of these advantages, there are also some disadvantages to the use of this polyaniline alignment layer. First of all, this well-known polyaniline film may corrode the transparent conducting glass (ITO glass) by the use of a strong acid electrolyte during the electrochemical polymerization. Secondly, since the adhesion between the conducting polymer and glass is low, there is a possibility that the conducting polymer film can be detached from the glass electrode. Third, when the liquid crystal device operates, the residual electrolyte ions or dopants in the polyaniline film can erupt into the liquid crystal, causing the shortening of the lifetime of the liquid crystal device.
Furthermore, due to the brittleness and the poor mechanical property of the polyaniline film, the polymer film can be detached from the ITO glass when the polymer surface is rubbed by a nylon comb to induce alignment of the liquid crystals.
Especially, it is not possible to synthesize an alignment layer with a large area by the electrochemical polymerization method. Thus, when an alignment layer is prepared by this method, there is a limitation in the size of the liquid crystal display device. In order to solve these problems, it is essential to use a conducting polymer with an excellent processability, which is accessible to the spin coating or dip coating technique. It is also desirable for the conducting polymer to have a liquid crystalline property to induce uniform alignment of liquid crystals on the surface of the alignment layer.
In particular, processing from an orientationally ordered polymer phase is capable of imparting a high degree of chain alignment and greatly enhanced mechanical, electrical, and optical properties.
Therefore, the inventors of the present application have developed a conducting polymer composite with excellent processability and synthesis technique, and which is effective in the preparation method of conductive alignment layers for LCDs.
Since liquid crystalline phase behavior is observed in the fluid state, liquid crystalline polymers are generally melt processible and/or solution processible.
However, conducting polymers such as polypyrrole, polyaniline, or polythiophene are not very soluble in a solvent, and decompose before melting. Thus it has not been easy to observe the liquid crystalline property from these polymers both in solution and in the fusion state.
One method is to synthesize the soluble conducting polymer to introduce alkyl groups or polar functional groups with long chains in the side chain of the conducting polymer.
One representative is poly(3-3'-thienyl propanesulfonic acid), which is soluble in water and shows lyotropic behavior in an aqueous solution (Mol. Cryst. Liq. Cryst., 1988, 160, 121).
But the redox stability of these thiophene polymers is low and synthesis of the substituted thiophene monomer requires many steps.
Among these conducting polymers, polyaniline has been attractive for practical use as an electronic material in a variety of applications such as a sensor, a photoelectric cell, an electrochromic display device, and a plastic storage battery, because polyaniline has a wide range of desirable electrical, electrochemical and optical properties, coupled with excellent environmental stability. Nonetheless, there is a weak point in that doped polyanilines in their conducting form cannot be directly used as an alignment layer due to its intractability.
Recently it was reported that the dissolution and lyotropic processing of polyaniline in its conducting form is made possible by doping polyaniline with camphor sulfonic acid (Polymer, 199, 34, 3139). However, solubility of the camphor sulfonic acid doped polyaniline in m-cresol is quite low, and it is very difficult to prepare a homogeneous lyotropic solution from this method. Another problem is that the preparation of the camphor sulfonic acid doped polyaniline requires extra steps of dedoping and redoping.
Therefore, the present invention has attempted to answer the above mentioned problems using the soluble conducting polymer obtained in the previous invention (Korean Patent Application No. 92-23786). Previously, we reported that this conducting polymer showed excellent redox stability and outstanding electrochemical and electrochromic stability. Thus, at this time we investigated the processability and liquid crystalline phase behavior of this polymer. It was found that the alkylsulfonate group attached to the side chain of the conducting polymer functions as a self-dopant of the oxidized polymer and as a rigidifying spacer of the polymer to form liquid crystals. The present invention was thus completed from the above finding.
In other words, the purpose of the present invention is to introduce the formation of a conducting polymer liquid crystal composite and its method of synthesis. This conducting polymer can be synthesized by using chemical and electrochemical techniques reported in the patent application and are providing with liquid crystal properties by being dissolved in a polar solvent and its mixture. Another purpose is to provide conducting polymer alignment layers and their method of preparation. These alignment layers do not produce a static charge during the fabrication of liquid crystal cells since the conductivity of the polymer film is as high as 10.sup.-3 s/cm.